1. Field Of The Invention
The present invention relates to a process for optimizing the combustion of an internal-combustion engine running under controlled self-ignition conditions.
2. Description of the Prior Art
Self-ignition is a well-known phenomenon in two-stroke engines which affords advantages concerning emissions since low hydrocarbon and nitrogen oxides emissions are notably obtained.
Concerning four-stroke engines, during self-ignition combustion, very low nitrogen oxides emissions can be obtained, as well as a remarkable cycle regularity.
Self-ignition is a phenomenon which allows initiation of the combustion by means of the residual burnt gases present in the combustion chamber after combustion.
This self-ignition is achieved by controlling the amount of residual burnt gases and mixing thereof with the fresh gases. The residual gases, which are hot burnt gases, initiate combustion of the fresh gases by means of a temperature combination and of the presence of active species (radicals).
In two-stroke engines, the presence of residual gases is inherent in the combustion.
In fact, when the load of the engine decreases, the amount of fresh gases decreases and the fresh gases are naturally replaced by an amount of residual burnt gases from the previous combustion cycle or cycles that have not left the cylinder.
The two-stroke engine thus runs with an internal recirculation (or internal EGR) of the burnt gases at partial load.
However, the presence of this internal EGR is not sufficient to obtain the desired self-ignition running; work on this subject has shown that mixing between this internal EGR and the fresh gases also has to be controlled and notably limited.
The controlled self-ignition technology applied to four-stroke engines is particularly interesting because it allows operation of the engine with an extremely diluted mixture, with very low fuel-air ratios and very low nitrogen oxides emissions.
A high-efficiency and low-pollution engine requires an optimum combustion, whatever the running conditions (speed, load, ambient temperature, hygrometry, ), aging and fouling of the engine.
The criteria to be optimized are mainly the combustion timing in the cycle and the rate of progress of this combustion.
In the case of a controlled self-ignition engine, the combustion is not initiated by a spark whose time of appearance can be controlled, but by the evolution of the thermodynamic and chemical conditions of the air and fuel mixing process during the compression stroke.
According to the variations of this evolution, the combustion can be adjusted early in the cycle and progress less quickly.
It is therefore necessary to adjust various combustion control parameters in order to permanently optimize the progress of this combustion. Document WO 99/40,296 proposes a process for operating a compression ignition engine that allows control of the air/fuel ratio of the mixture present in the combustion chamber by modifying the compression ratio by means of an adjustable intake element such as the intake valve with which such an engine is usually equipped.
The state of the combustion is therefore measured, then the closure time of the intake valve of the combustion chamber is adjusted for the next cycle according to the signal resulting from this measurement.
The process described in this document only allows adjustment of the compression alone or associated with other parameters, and for the next cycle, which implies adjustments of these parameters whose response time is shorter than the combustion cycle length.
The present invention is a process for optimizing the combustion, which accounts for certain parameters necessary to obtain an ideal combustion and their specific response times.
The process according to the invention is therefore characterized in that:
several parameters to be adjusted are determined to optimize the combustion,
the parameters are divided into fast parameters and slow parameters,
the fast parameters are managed by means of a control loop specific to the parameters and the slow parameters are managed by means of a control loop specific to the slow parameters to obtain the desired combustion for the next cycles.
By means of this process, all of the pertinent parameters in the combustion chamber of the controlled self-ignition engine can be controlled by accounting for their specific response times as regards combustion.
These parameters are notably the amount of air admitted, the amount of fuel, the global and local air/fuel ratio, the amount of internal burnt gases and the degree of mixing of internal burnt gases with the fresh gases, the dilution of the fresh gases by external recirculated burnt gases, . . . etc.
More particularly, the slow parameters are controlled and the fast parameters are controlled by accounting for the slow parameters control.
By means of this characteristic, the inadequate adjustment of the slow parameters can be compensated by the suitable adjustment of the fast parameters, which eventually allows obtaining the desired combustion, whatever the running conditions.
More particularly, the slow parameters have a longer response time than the fast parameters.
The response time of the fast parameters is preferably at most equal to the length of a determined number of combustion cycles.
According to the invention, the parameters to be adjusted are determined by comparison of a reference frame for an ideal combustion contained in the unit and processing of the signals sent by at least one combustion state detector.
More particularly, the signals come from a combustion state detector and/or from a knock detector and/or from a pressure detector.
According to an aspect of the invention, the processing unit corrects the slow parameters so that the fast parameters remain within boundary values.
Thus, as the engine is operated and as it evolves with time (fouling, aging), the control process automatically corrects certain slow parameters such as, for example, the burnt gas recirculation, so that the variations of the fast parameters remain within acceptable predetermined boundary values (such as the opening and closure time of the valves in the case of an electromechanical or electrohydraulic type valve gear).